Reassortment and mutation of the avian influenza polymerase PA subunit overcomes species barriers

Andrew Mehle1,*,
Vivien G. Dugan2,
Jeffery K. Taubenberger2 and
Jennifer A. Doudna1,3,4

+ Author Affiliations

ABSTRACT

Emergence of new pandemic influenza A viruses requires overcoming barriers to cross-species transmission as viruses move from animal reservoirs into humans. This complicated process is driven by both individual gene mutations and genome reassortment.

The viral polymerase complex, composed of the proteins PB1, PB2 and PA, is a major viral factor controlling host-adaptation, and reassortment events involving polymerase gene segments occurred in past pandemic viruses.

Here we investigate the ability of polymerase reassortment to restore the activity of an avian influenza polymerase that is normally impaired in human cells.
Our data show that substitution of human-origin PA subunits into an avian influenza polymerase alleviates restriction in human cells and increases polymerase activity in vitro. Reassortants with 2009 pandemic H1N1 PA proteins were the most active.

Mutational analyses demonstrate that the majority of the enhancing activity in human PA results from a threonine to serine change at residue 552. Reassortant viruses with avian polymerases and human PA subunits, or simply the T552S mutation, displayed faster replication kinetics in culture and increased pathogenicity in mice compared to those containing a wholly avian polymerase complex. Thus, acquisition of a human PA subunit, or the signature T552S mutation, is a potential mechanism to overcome the species-specific restriction of avian polymerases and increase virus replication.
Our data suggest that the human, avian, swine and 2009 H1N1-like viruses that are currently co-circulating in pig populations set the stage for PA reassortments with the potential to generate novel viruses that could possess expanded tropism and enhanced pathogenicity.